Anode active material and battery

ABSTRACT

Anode active materials, anodes, and batteries are provided. In one embodiment, an anode active material includes particles consisting essentially of a material selected from the group consisting of silicon and an alloy of silicon. An average degree of circularity of the particles is 90% or less.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.11/555,098, filed on Oct. 31, 2006, which claims priority to JapanesePatent Application JP 2005-321015 filed in the Japanese Patent Office onNov. 4, 2005, the entire contents of which being incorporated herein byreference.

BACKGROUND

The present disclosure relates to an anode active material containing atleast one of metal elements and metalloid elements as an element and abattery using the anode.

In recent years, as mobile devices have been sophisticated andmulti-functionalized, a higher capacity of secondary batteries as apower source for these mobile devices has been demanded. As a secondarybattery to meet such a demand, there is a lithium ion secondary battery.However, since graphite is used for the anode in the lithium ionsecondary battery in practical use currently, the battery capacitythereof is in a saturated state and thus it is difficult to attain avastly high capacity thereof. Therefore, it is considered to use silicon(Si), tin (Sn), an alloy thereof or the like as an anode active materialcapable of attributing to a higher capacity (for example, refer toJapanese Unexamined Patent Application Publication No. 2000-311681).

However, in the anode active material using silicon or tin as above, thevolume is largely changed associated insertion and extraction of lithium(Li) and the shape is deformed. Therefore, there has been a problem thatthe charge and discharge characteristics are lower than those of carbonmaterials, and it is difficult to utilize its characteristics of thehigh capacity.

SUMMARY

In view of the foregoing, it is desirable to provide an anode activematerial capable of preventing shape deformation due to expansion andshrinkage and a battery using the anode.

According to an embodiment of the present disclosure, an anode activematerial includes particles consisting essentially of a materialselected from the group consisting of silicon and an alloy of silicon.An average degree of circularity of the particles is 90% or less.

According to another embodiment of the present disclosure, an anodeincludes an anode current collector and an anode active material layerprovided on the anode current collector. The anode active material layerincludes particles consisting essentially of a material selected fromthe group consisting of silicon and an alloy of silicon. An averagedegree of circularity of the particles is 90% or less.

According to another embodiment of the present disclosure, a batteryincludes a cathode, an anode, and an electrolytic solution. The anodecontains an anode active material including particles consistingessentially of a material selected from the group consisting of siliconand an alloy of silicon. An average degree of circularity of theparticles is 90% or less.

According to another embodiment of the present disclosure, a batteryincludes a cathode, an anode, and an electrolytic solution. The anodecontains an anode current collector and an anode active materialprovided on the anode current collector. The anode active materialincludes particles consisting essentially of a material selected fromthe group consisting of silicon and an alloy of silicon. An averagedegree of circularity of the particles is 90% or less. A surfaceroughness Ra of the anode current collector ranges from 0.1 μm to 3.5μm.

According to the anode active material of the embodiment of theinvention, the average degree of circularity is 0.90 or less. Therefore,the surface area becomes wide, and the reactive region becomes large. Inthe result, local intense reaction is prevented, and cracks resultingfrom expansion and shrinkage can be prevented. Therefore, according tothe battery of the embodiment of the invention using the anode activematerial, shape deformation of the anode can be prevented, and superiorcharge and discharge characteristics can be obtained.

In particular, when the average particle diameter of the anode activematerial is 20 μm or less, higher effects can be obtained.

Additional features and advantages are described herein, and will beapparent from, the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a partial exploded perspective view showing a structure of asecondary battery according to an embodiment; and

FIG. 2 is a cross section showing a structure taken along line I-I of aspirally wound electrode body shown in FIG. 1.

DETAILED DESCRIPTION

An embodiment of the invention will be hereinafter described in detailwith reference to the drawings.

An anode active material according to an embodiment is granular, andcontains, for example, at least one of metal elements and metalloidelements capable of forming an alloy with lithium as an element. As ametal element or a metalloid element capable of forming an alloy withlithium, for example, magnesium (Mg), boron (B), arsenic (As), aluminum(Al), gallium (Ga), indium (In), silicon (Si), germanium (Ge), tin (Sn),lead (Pb), antimony (Sb), bismuth (Bi), cadmium (Cd), silver (Ag), zinc(Zn), hafnium (Hf), zirconium (Zr), yttrium (Y), palladium (Pd), orplatinum (Pt) can be cited. Specially, metal elements or metalloidelements in Group 14 in the long period periodic table are preferable.Silicon or tin is particularly preferable, since thereby a highercapacity can be obtained.

The anode active material may contain a simple substance, an alloy, or acompound of the metal elements or the metalloid elements. Further, theanode active material may have one or more phases thereof at least inpart. In an embodiment, alloys include an alloy of one or more metalelements and one or more metalloid elements in addition to an alloy oftwo or more metal elements. Further, the alloy may contain a nonmetallicelement. The texture thereof may be a solid solution, a eutectic crystal(eutectic mixture), an intermetallic compound, or a texture in which twoor more thereof exist.

The average degree of circularity of the anode active material ispreferably 0.90 or less, more preferably 0.85 or less, much morepreferably 0.80 or less, and most preferably 0.75 or less. When thedegree of circularity is high, intense reaction is generated on thesurface, and cracks easily occur when expanded and shrunk. By decreasingthe degree of circularity, the surface area becomes wide, the reactiveregion becomes large, and thus lithium is inserted more uniformly. Inthe result, volume change becomes more uniform, and the number of cracksresulting from expansion and shrinkage are reduced. The degree ofcircularity can be obtained by, for example, calculating a peripherallength and an area from a projected image of a particle, and usingMathematical formula 1.Degree of circularity=perimeter of a circle having the same area as thatof a projected image of a particle/perimeter of the projected image ofthe particle  Mathematical Formula 1

The average particle diameter of the anode active material is preferably20 μm or less, and more preferably 10 μm or less. Thereby, the surfacearea becomes wide, and higher effects can be obtained. The averageparticle diameter of the anode active material is preferably 0.3 μm ormore, and more preferably 0.5 μm or more. There is a tendency that whenthe particle diameter is smaller, cracks are hard to be generated.However, when the particles are excessively small and the surface areaexcessively increases, decomposition reaction of the electrolyte becomesintense.

The degree of circularity of the anode active material can be adjustedby a process such as pulverization. When the particles of the anodeactive material are formed by vapor-phase deposition method, the degreeof circularity of the anode active material can be adjusted bycontrolling conditions of forming the particles of the anode activematerial. The average particle diameter of the anode active material canbe adjusted by classification or the like.

The anode active material is used for secondary batteries as follows,for example.

FIG. 1 shows a structure of the secondary battery. In the secondarybattery, a spirally wound electrode body 10 on which leads 11 and 12 areattached is contained inside a film package member 21. The leads 11 and12 are respectively directed from inside to outside of the packagemember 21 and derived in the same direction, for example. The leads 11and 12 are respectively made of, for example, a metal material such asaluminum, copper, nickel, and stainless, and are in a state of a thinplate or mesh, respectively.

The package member 21 is made of a rectangular aluminum laminated filmin which, for example, a nylon film, an aluminum foil, and apolyethylene film are bonded together in this order. The package member21 is, for example, arranged so that the polyethylene film side and thespirally wound electrode body 10 are opposed to each other, and therespective outer edges are contacted to each other by fusion bonding oran adhesive. Adhesive films 22 to protect from entering of outside airare inserted between the package member 21 and the leads 11 and 12. Theadhesive film 22 is made of a material having contact characteristics tothe leads 11 and 12, for example, a polyolefin resin such aspolyethylene, polypropylene, modified polyethylene, and modifiedpolypropylene.

The package member 21 may be made of a laminated film having otherstructure, a polymer film such as polypropylene, or a metal film,instead of the foregoing aluminum laminated film.

FIG. 2 shows a cross sectional structure taken along line I-I of thespirally wound electrode body 10 shown in FIG. 1. In the spirally woundelectrode body 10, an anode 13 and a cathode 14 are layered and spirallywound with a separator 15 in between. The outermost periphery thereof isprotected by a protective tape 16.

The anode 13 has an anode current collector 13A and an anode activematerial layer 13B provided on the anode current collector 13A. Theanode current collector 13A is preferably made of a metal materialcontaining at least one of metal elements not forming an intermetalliccompound with lithium. When the intermetallic compound is formed withlithium, the anode current collector is expanded and shrunk due tocharge and discharge, structural deformation occurs, and the currentcollectivity is lowered. In addition, an ability to support the anodeactive material layer 13B becomes lowered, and thus the anode activematerial layer 13B becomes easily dropped from the anode currentcollector 13A. As a metal element not forming an intermetallic compoundwith lithium, for example, copper (Cu), nickel (Ni), titanium (Ti), iron(Fe), or chromium (Cr) can be cited. Further, as a metal materialcomposing the anode current collector 13A, a material containing a metalelement which can be alloyed with the anode active material layer 13B ispreferable. Thereby, the contact characteristics between the anodeactive material layer 13B and the anode current collector 13A can beimproved.

The surface roughness Ra of the current collector 13A is preferably 0.1μm or more. Thereby, the contact characteristics between the anodeactive material layer 13B and the current collector 13A can be moreimproved. Further, the surface roughness Ra of the current collector 13Ais preferably 3.5 μm or less, and more preferably 3.0 μm or less. Whenthe surface roughness Ra is excessively high, there is a possibilitythat cracks are easily generated in the anode current collector 13A dueto expansion of the anode active material layer 13B. The surfaceroughness Ra is arithmetic average roughness Ra specified in JIS B0601.It is enough that, the surface roughness Ra of at least a region of theanode current collector 13A on which the anode active material layer 13Bis provided falls within the foregoing range.

The anode active material layer 13B contains the anode active materialaccording to this embodiment. If necessary, the anode active materiallayer 13B may contain other materials such as an electrical conductor, abinder, or other anode active material. The average particle diameter ofthe anode active material according to this embodiment is preferablywithin the foregoing range in a full discharge state. Further, in theanode active material, particles may be chemically bonded with eachother at the contact points, or may be physically contacted with eachother simply, or may be adhered to each other by a binder. Further, theanode active material layer 13B may be alloyed with the anode currentcollector 13A in at least part of the interface with the anode currentcollector 13A. Thereby, the contact characteristics between the anodeactive material layer 13B and the anode current collector 13A can beimproved. Specifically, in the interface, the element of the anodecurrent collector 13A may be diffused in the anode active material layer13B, or the element of the anode active material layer 13B may bediffused in the anode current collector 13A, or the both elementsthereof may be diffused in each other. In this application, theforegoing diffusion of elements is regarded as one form of alloying.

The cathode 14 has, for example, a cathode current collector 14A and acathode active material layer 14B provided on the cathode currentcollector 14A. The cathode current collector 14A is made of, forexample, aluminum, nickel, or stainless steel.

The cathode active material layer 14B contains, for example, as acathode active material, one or more cathode materials capable ofinserting and extracting lithium. The cathode active material layer 14Bmay contain an electrical conductor such as a carbon material and abinder such as polyvinylidene fluoride according to needs. As a cathodematerial capable of inserting and extracting lithium, for example, alithium-transition metal complex oxide containing lithium and transitionmetals is preferable, since thereby a high voltage can be generated anda high energy density can be obtained. As the lithium-transition metalcomplex oxide, for example, a compound expressed by a general formula,Li_(x)MO₂ can be cited. M includes one or more transition elements, andfor example, preferably includes at least one of cobalt and nickel. xvaries according to charge and discharge states of the battery, and isgenerally in the range of 0.05≦x≦1.10. As a specific example of such alithium-transition metal complex oxide, LiCoO₂, LiNiO₂ or the like canbe cited.

The separator 15 separates the anode 13 from the cathode 14, preventscurrent short circuit due to contact of the both electrodes, and allowspassage of lithium ions. The separator 15 is made of, for example,polyethylene or polypropylene.

An electrolytic solution which is a liquid electrolyte is impregnated inthe separator 15. The electrolytic solution contains, for example, asolvent and an electrolyte salt dissolved in the solvent. Theelectrolytic solution may contain an additive according to needs. As asolvent, for example, a nonaqueous solvent such as ethylene carbonate,propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methylcarbonate, 1,3-dioxole-2-one, 4-vinyl-1,3-dioxolane-2-one, and an estercarbonate derivative having halogen atom can be cited. One of thesolvents may be used singly, or a mixture of two or more solvents may beused. Specially, at least one of 1,3-dioxole-2-one and4-vinyl-1,3-dioxolane-2-one is preferably used, since therebydecomposition reaction of the electrolytic solution can be prevented.The ester carbonate derivative having halogen atom is also preferablyused, since thereby decomposition reaction of the electrolytic solutioncan be prevented.

The ester carbonate derivative having halogen atom may be a cycliccompound or a chain compound. However, the cyclic compound ispreferable, since thereby higher effects can be obtained. As such acyclic compound, 4-fluoro-1,3-dioxolane-2-one,4-chloro-1,3-dioxolane-2-one, 4-bromo-1,3-dioxolane-2-one,4,5-difluoro-1,3-dioxolane-2-one or the like can be cited. Specially,4-fluoro-1,3-dioxolane-2-one is preferable, since thereby higher effectscan be obtained.

As an electrolyte salt, for example, a lithium salt such as LiPF₆,LiCF₃SO₃, and LiClO₄ can be cited. One of the electrolyte salts may beused singly, or two or more thereof may be used by mixing.

The electrolytic solution may be directly used as it is, or may be usedas a so-called gelatinous electrolyte by being held by a polymer. In thelatter case, the electrolyte may be impregnated in the separator 15, ormay exist as a layer between the separator 15 and the anode 13/thecathode 14. As a polymer material, for example, a polymer containingvinylidene fluoride is preferable, since such a polymer has high redoxstability. Further, as a polymer, a compound formed by polymerizing apolymeric compound is preferable. As a polymeric compound, for example,monofunctional acrylate such as acrylic ester, monofunctionalmethacrylate such as methacrylic ester, multifunctional acrylate such asdiacrylic ester and triacrylic ester, multifunctional methacrylate suchas dimethacrylic ester and trimethacrylic ester, acrylic nitrile,methacrylic nitrile can be cited. Specially, ester having an acrylategroup or a methacrylate group is preferable, since when such ester isused, polymerization proceeds easily and the reaction rate of thepolymeric compound is high.

The battery can be manufactured, for example, as follows.

First, for example, the anode active material according to thisembodiment, and if necessary an electrical conductor, a binder or thelike are mixed to prepare a mixture. The mixture is dispersed in adisperse medium to obtain mixture slurry. The anode current collector13A is coated with the mixture slurry. The resultant iscompression-molded to form the anode active material layer 13B. Next, ifnecessary, heat treatment may be provided to alloy the anode currentcollector 13A and the anode active material layer 13B in at least partof the interface thereof. Thereby, the anode 13 is formed.

Further, for example, the cathode active material, and if necessary anelectrical conductor, a binder or the like are mixed to prepare amixture. The mixture is dispersed in a disperse medium to obtain mixtureslurry. The cathode current collector 14A is coated with the mixtureslurry. The resultant is compression-molded to form the cathode activematerial layer 14B. Thereby, the cathode 14 is formed.

Next, the anode 13 and the cathode 14 are spirally wound with theseparator 15 in between and such a spirally wound body is sandwichedbetween the package members 21. After that, an electrolytic solution isinjected into the package member 21, and then the package member 21 ishermetically sealed. Thereby, the secondary battery shown in FIGS. 1 and2 is completed. Otherwise, when the electrolytic solution is held by apolymer, a polymeric compound is injected together with the electrolyticsolution into the package member 21, the polymeric compound ispolymerized inside the package member 21, and thereby a gelatinouselectrolyte may be obtained. Otherwise, it is possible that before theanode 13 and the cathode 14 are spirally wound with the separator 15 inbetween, an electrolyte in which an electrolytic solution is held in apolymer is formed on the anode 13 or the cathode 14.

In the secondary battery, when charged, for example, lithium ions areextracted from the cathode 14 and inserted in the anode 13 through theelectrolytic solution. When discharged, for example, lithium ions areextracted from the anode 13 and inserted in the cathode 14 through theelectrolytic solution. Here, since the average degree of circularity ofthe anode active material is 0.90 or less, the reactive region becomeslarge, and intense local reaction is prevented. Therefore, even if theanode active material is expanded and shrunk due to charge anddischarge, shape deformation such as cracks is prevented.

As above, according to this embodiment, since the average degree ofcircularity of the anode active material is 0.90 or less, the surfacearea becomes large, the reactive region becomes large, and intense localreaction is prevented. Thus, cracks resulting from expansion andshrinkage can be prevented. Therefore, shape deformation of the anodeactive material layer 13B can be prevented, and superior charge anddischarge characteristics can be obtained.

In particular, when the average degree of circularity of the anodeactive material is 0.85 or less, preferably 0.80 or less, morepreferably 0.75 or less, improved effects can be obtained.

Further, when the average particle diameter of the anode active materialis 20 μm or less, further 10 μm or less, and when the average particlediameter of the anode active material is 0.3 μm or more, preferably 0.5μm or more, improved effects can be obtained.

EXAMPLES

Further, specific examples will be hereinafter described in detail withreference to the drawings. In the following examples, reference numbersand symbols used in the foregoing embodiments are used correspondingly.

Examples 1-1 to 1-15

In Examples 1-1 to 1-3, the laminated film secondary batteries shown inFIGS. 1 and 2 were fabricated by using crystalline silicon particles asan anode active material. At that time, the crystalline siliconeparticles were pulverized, thereby changing the average degree ofcircularity to 0.90, 0.85, or 0.80 in Examples 1-1 to 1-3. The averagedegree of circularity was calculated according to Mathematical formula 1by using a flow system particle image analyzer. The average degree ofcircularity was also calculated similarly in examples and comparativeexamples described later. Further, the average particle diameter of thecrystalline silicon particles was adjusted to 5 μm by classification.

In Examples 1-4 to 1-9, the laminated film secondary batteries shown inFIGS. 1 and 2 were fabricated by using amorphous silicon particles as ananode active material. The amorphous silicone particles were formed on amolybdenum substrate with the surface roughened by electron beam vapordeposition method by vapor-phase freezing. At that time, the vapordeposition conditions were adjusted, thereby changing the average degreeof circularity of the amorphous silicon particles to 0.90, 0.85, 0.80,0.75, 0.70, or 0.65 in Examples 1-4 to 1-9. Further, the averageparticle diameter of the amorphous silicon particles was adjusted to 4μm by classification.

In Examples 1-10 to 1-12, the laminated film secondary batteries shownin FIGS. 1 and 2 were fabricated by using as an anode active material, amixture of crystalline silicon particles and graphitized meso-carbonmicro beads powder as artificial graphite at a weight ratio ofcrystalline silicon particles:artificial graphite=50:40. At that time,the crystalline silicone particles were pulverized, thereby changing theaverage degree of circularity to 0.90, 0.85, or 0.80 in Examples 1-10 to1-12. The average particle diameter of the crystalline silicon particleswas adjusted to 5 μm by classification. Further, the average degree ofcircularity of the artificial graphite particles was 0.98, and theaverage particle diameter of the artificial graphite particles was 20μm.

In Examples 1-13 to 1-15, the laminated film secondary batteries shownin FIGS. 1 and 2 were fabricated by using silicon-titanium-copper alloyparticles as an anode active material. The silicon-titanium-copper alloyparticles were synthesized by single-roll process. The compositionthereof was 60:30:10 at an atomic ratio of Si:Ti:Cu. Then, pulverizationconditions were changed, thereby changing the average degree ofcircularity to 0.90, 0.85, or 0.80 in Examples 1-13 to 1-15. The averageparticle diameter was adjusted to 10 μm by classification.

In Examples 1-1 to 1-15, the anode 13 was formed as follows. 90 wt % ofan anode active material and 10 wt % of polyvinylidene fluoride as abinder were mixed, the resultant mixture was dispersed in a solvent. Theanode current collector 13A made of a 15 μm thick copper foil with asurface roughness Ra of 0.3 μm was coated with the foregoing resultant,which was pressed to form the anode active material layer 13B. Afterthat, heat treatment was provided at 200° C. in the argon atmosphere toform the anode 13.

The cathode 14 was formed as follows. Lithium cobaltate (LiCoO₂) as acathode active material, carbon black as an electrical conductor, andpolyvinylidene fluoride as a binder were mixed. The resultant mixturewas dispersed in a disperse medium. After that, the cathode currentcollector 14A made of an aluminum foil was coated with the foregoingresultant, which was pressed to form the cathode active material layer14B. Thereby, the cathode 14 was formed.

Further, a battery was assembled as follows. The anode 13 and thecathode 14 were layered and spirally wound with a microporouspolyethylene film as the separator 15 in between. After that, theresultant spirally wound body was sandwiched between the package members21 made of an aluminum laminated film. An electrolytic solution wasinjected into the package member 21, the package member 21 washermetically sealed, and thereby the battery was assembled. As anelectrolytic solution, a solution in which LiPF₆ as an electrolyte saltwas dissolved at a concentration of 1 mol/l in a mixture of ethylenecarbonate and diethyl carbonate at a weight ratio of ethylenecarbonate:diethyl carbonate=3:7 was used.

As Comparative example 1-1 relative to Examples 1-1 to 1-15, a secondarybattery was fabricated in the same manner as in Examples 1-1 to 1-3,except that the average degree of circularity of the crystalline siliconparticles as an anode active material was 0.95.

Further, as Comparative examples 1-2 to 1-4 relative to Examples 1-1 to1-15, secondary batteries were fabricated in the same manner as inExamples 1-1 to 1-15, except that natural graphite particles were usedas an anode active material. At that time, the natural graphiteparticles were pulverized, thereby changing the average degree ofcircularity to 0.90, 0.85, or 0.80 in Comparative examples 1-2 to 1-4.Further, the average particle diameter of the natural graphite particleswas adjusted to 25 μm by classification.

For the fabricated secondary batteries of Examples 1-1 to 1-15 andComparative examples 1-1 to 1-4, cycle test was performed. In the cycletest, 50 cycles of constant current and constant voltage charge with acurrent of 500 mA and the upper limit voltage of 4.2 V and constantcurrent discharge with a current of 500 mA and the lower limit voltageof 2.5 V were performed at room temperature. The ratio of the dischargecapacity at the 50th cycle to the discharge capacity at the first cycle,that is, (the discharge capacity at the 50th cycle/the dischargecapacity at the first cycle)×100(%) was examined. Obtained results areshown in Table 1.

TABLE 1 Anode active material Dis- Average Average charge degreeparticle capacity of diameter retention Kind circularity (μm) ratio (%)Example 1-1 Crystalline 0.90 5 77 Example 1-2 Si 0.85 79 Example 1-30.80 83 Example 1-4 Amorphous 0.90 4 76 Example 1-5 Si 0.85 82 Example1-6 0.80 84 Example 1-7 0.75 87 Example 1-8 0.70 89 Example 1-9 0.65 91Example 1-10 Crystalline Si: Si = 0.90 Si = 5 76 Artificial ArtificialArtificial graphite = graphite = 0.98 graphite = 20 50:40 Example 1-11(weight ratio) Si = 0.85 79 Artificial graphite = 0.98 Example 1-12 Si =0.80 84 Artificial graphite = 0.98 Example 1-13 SiTiCu alloy 0.90 10 80Example 1-14 Si:Ti:Cu = 0.85 83 Example 1-15 60:30:10 0.80 87 (atomic %)Comparative Crystalline Si 0.95 5 67 example 1-1 Comparative Natural0.90 25 91 example 1-2 graphite Comparative 0.85 87 example 1-3Comparative 0.80 86 example 1-4

As shown in Table 1, according to Examples 1-1 to 1-15 in which theaverage degree of circularity was 0.90 or less, the discharge capacityretention ratio could be improved compared to Comparative example 1-1 inwhich the average degree of circularity was higher than 0.90. Further,in Examples 1-1 to 1-15, there was a tendency that as the average degreeof circularity was lowered, the discharge capacity retention ratioimproved. Meanwhile, in Comparative examples 1-2 to 1-4 in which thenatural graphite particles were used as an anode active material, therewas a tendency that as the average degree of circularity was lowered,the discharge capacity retention ratio was lowered.

That is, it was found that when the average degree of circularity was0.90 or less in the powdery anode active material containing at leastone of metal elements and metalloid elements as an element, the cyclecharacteristics could be improved. Further, it was found that theaverage degree of circularity was preferably 0.85 or less, morepreferably 0.80 or less, and much more preferably 0.75 or less.

Examples 2-1 to 2-3

Secondary batteries were fabricated in the same manner as in Examples1-1 to 1-15, except that tin-copper-cobalt alloy particles were used asan anode active material. The tin-copper-cobalt alloy particles weresynthesized by single-roll process. The composition thereof was 50:30:20at an atomic ratio of Sn:Cu:Co. The average degree of circularity waschanged to 0.90, 0.85, or 0.80 by pulverization in Examples 2-1 to 2-3.The average particle diameter was adjusted to 12 μm by classification.

As Comparative example 2-1 relative to Examples 2-1 to 2-3, a secondarybattery was fabricated in the same manner as in Examples 2-1 to 2-3,except that the average degree of circularity of the tin-copper-cobaltalloy particles as an anode active material was 0.95. For the fabricatedsecondary batteries of Examples 2-1 to 2-3 and Comparative example 2-1,cycle test was performed in the same manner as in Examples 1-1 to 1-15.Obtained results are shown in Table 2 together with the results ofComparative examples 1-2 to 1-4.

TABLE 2 Anode active material Discharge Average Average capacity degreeof particle retention Kind circularity diameter (μm) ratio (%) Example2-1 SnCuCo alloy 0.90 12 74 Example 2-2 Sn:Cu:Co = 0.85 80 Example 2-350:30:20 0.80 85 (atomic %) Comparative SnCuCo alloy 0.95 12 69 example2-1 Sn:Cu:Co = 50:30:20 (atomic %) Comparative Natural graphite 0.90 2591 example 1-2 Comparative 0.85 87 example 1-3 Comparative 0.80 86example 1-4

As shown in Table 2, in Examples 2-1 to 2-3, there was also a tendencythat as the average degree of circularity was lowered, the dischargecapacity retention ratio improved as in Examples 1-1 to 1-15. That is,it was found that when the average degree of circularity was 0.90 orless, the cycle characteristics could be also improved in the case thatother anode active material containing at least one of metal elementsand metalloid elements as an element was used.

Examples 3-1 to 3-9

In Examples 3-1 to 3-9, secondary batteries were fabricated in the samemanner as in Example 1-2, except that the average particle diameter ofthe crystalline silicon particles as an anode active material waschanged in the range from 0.3 μm to 30 μm. In Examples 3-7 to 3-9,secondary batteries were fabricated in the same manner as in Example2-2, except that the average particle diameter of the tin-copper-cobaltalloy particles as an anode active material was changed in the rangefrom 8 μm to 30 μm.

Further, as Comparative examples 3-1 and 3-2 relative to Examples 3-1 to3-9, secondary batteries were fabricated in the same manner as inExamples 3-1 to 3-9, except that natural graphite particles with theaverage degree of circularity of 0.85 was used as an anode activematerial, and the average particle diameter was 30 μm or 10 μm.

For the fabricated secondary batteries of Examples 3-1 to 3-8 andComparative examples 3-1 and 3-2, cycle test was performed in the samemanner as in Examples 1-2 and 2-2. Obtained results are shown in Table 3together with the results of Examples 1-2 and 2-2, and Comparativeexample 1-3. For the secondary batteries of Examples 3-1 to 3-8 andComparative examples 3-1 and 3-2, after full discharged state wasobtained by performing 1 cycle of charge and discharge under theforegoing conditions, the secondary batteries were disassembled, and theaverage particle diameter of the anode active material was measured. Inthe result, it was confirmed that the measured average particle diameterwas almost identical with the average particle diameter adjusted inmanufacturing.

TABLE 3 Anode active material Discharge Average Average capacity degreeparticle retention Kind of circularity diameter (μm) ratio (%) Example3-1 Crystalline Si 0.85 30 70 Example 3-2 20 75 Example 3-3 10 76Example 1-2 5 79 Example 3-4 2 82 Example 3-5 0.5 81 Example 3-6 0.3 73Example 3-7 SnCuCo alloy 0.85 30 69 Example 3-8 Sn:Cu:Co = 20 75 Example2-2 50:30:20 12 80 Example 3-9 (atomic %) 8 83 Comparative Naturalgraphite 0.85 30 88 example 3-1 Comparative 25 87 example 1-3Comparative 10 84 example 3-2

As shown in Table 3, in Examples 3-1 to 3-9, there was a tendency thatas the average particle diameter was decreased, the discharge capacityretention ratio improved and then lowered. Meanwhile, in Comparativeexamples 1-3, 3-1, and 3-2, there was a tendency that as the averageparticle diameter was decreased, the discharge capacity retention ratiowas lowered. That is, it was found that when the average particlediameter was 20 μm or less, further 10 μm or less, and when the averageparticle diameter was 0.3 μm or more, further 0.5 μm or more in thepowdery anode active material containing at least one of metal elementsand metalloid elements as an element, the cycle characteristics could bemore improved.

It should be appreciated that the invention is not limited to theforegoing embodiments and examples, and various modifications may bemade. For example, in the foregoing embodiment and the foregoingexamples, descriptions have been given of the case using the filmpackage member 21. However, other embodiments can be similarly appliedto a case using a can as a package member. The shape thereof may be anyshape such as a coin type, a cylinder type, a square type, a buttontype, a thin type, and a large type.

Further, in the foregoing embodiment and the foregoing examples,descriptions have been given of the case including the spirally woundelectrode body 10 in which the anode 13 and the cathode 14 are spirallywound. However, other embodiments can be similarly applied to alaminated type in which one or a plurality of anode layers and one or aplurality of cathode layers are layered. Further, other embodiments canbe applied to primary batteries in addition to the secondary batteries.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alternations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present subjectmatter and without diminishing its intended advantages. It is thereforeintended that such changes and modifications be covered by the appendedclaims.

The invention is claimed as follows:
 1. An anode active materialcomprising: particles consisting essentially of silicon, wherein anaverage degree of circularity of the particles is less than 70%, and anaverage particle diameter is 0.3 μm to 20 μm.
 2. The anode activematerial according to claim 1, wherein the average particle diameter is0.3 μm to 10 μm.
 3. The anode active material according to claim 1,further comprising a carbon material.
 4. The anode active materialaccording to claim 1, wherein the particles are monolithic particles. 5.An anode comprising: an anode current collector; and an anode activematerial layer provided on the anode current collector, the anode activematerial layer comprising particles consisting essentially of silicon,wherein an average degree of circularity of the particles is less than70%, and an average particle diameter is 0.3 μm to 20 μm.
 6. The anodeaccording to claim 5, wherein a surface roughness Ra of the anodecurrent collector ranges from 0.1 μm to 3.5 μm.
 7. A battery comprising:a cathode; an anode; and an electrolytic solution, wherein the anodecontains an anode active material comprising particles consistingessentially of silicon, and wherein an average degree of circularity ofthe particles is less than 70%, and an average particle diameter is 0.3μm to 20 μm.
 8. The battery according to claim 7, wherein a dischargecapacity retention ratio of the battery increases as the average degreeof circularity of the particles is lowered.
 9. An electronic deviceincluding a battery as recited in claim
 7. 10. The electronic deviceaccording to claim 9, wherein the electronic device is a mobile device.